D&D 5E Space, squares and reach with 3D combat

[Kobold Avenger]

So I realized that "space" for creatures is only 2D, I assume a medium or small creature takes up a 5x5x5 foot space and one on the ground can attack flying creatures that are 5 feet off the ground in any of those 9 cubes that are above their space. If a medium or small creature is flying or underwater, I assume they can also attack any of the 9 cubes that are under them.

So going by this should large creatures take up a 10x10x10 space and a large creature on the ground be able to melee attack creatures in cubes that are within 20 feet of the ground? Potentially can a large creature standing on a street, attack someone standing on the second floor (assuming that 1 floor/story is 10 feet in height) depending if they're on something like a balcony or by a window? Or a huge creature being able to reach the third floor with an attack where they're standing?

For those being attacked by those large creatures standing on the street, does being in a building count as having cover? Or does the large creature's attacks have disadvantage in many of these cases?

 

[wedgeski]

For me, the answer to this post is "yes". These are all safe, intuitive judgements that can greatly improve a combat. To your last question, I would grant cover, not disadvantage.

Verticality can be a massive bind. We tend to heavily abstract the third dimension. This causes the odd, brief negotiation both from the players' perspective and mine, but often, you can intuitively rewind a combat to show that someone is in range of something based on what they did last turn.

 

[ezo]

We play that reach and space are cubes, since 5E basically ignores the diagonals.

In your examples, being inside a room while attacked by a large creature outside would definitely provide cover (at least half, and likely 3/4-cover). However, you have to consider reach. Even most large creatures only have a reach of 5 feet, so a large-creature would not normally be able to hit a target 20 feet high.

 

[EzekielRaiden]

Verticality can be a massive bind. We tend to heavily abstract the third dimension.

Humans are much better at thinking two-dimensionally rather than three-dimensionally. Probably because our survival skills involved navigating the two-dimensional surface we lived, foraged, fled, and hunted on, or a relatively thin canopy layer. It takes training to think in 3D. This is part of why, even though most video games today are 3D spaces, levels are either flat planes with small deviations, or only make very limited use of actual 3D space; the typical player will make far more random errors when forced to think in full 3D, and will thus usually have less fun unless their skills are first given the chance to slowly improve over time.